This invention is directed to a nonwoven battery separator for a nickel-metal hydride (NiMH) battery.
A nonwoven separator for a nickel-metal hydride battery is known. See, for example, U.S. Pat. Nos. 5,589,302 and 5,830,604, both incorporated herein by reference.
Nickel-metal hydride (NiMH) batteries are known. See: Linden, D., Ed., Handbook of Batteries, 2nd Ed., McGraw-Hill, Inc. (1995), Chapter 33; and Besenhard, J. O., Ed., Handbook of Battery Materials, Wiley-VCH (1999), xc2xa7xc2xa79.3.3.2 and 9.3.5, both are incorporated herein by reference. In general, NiMH batteries are an improvement over nickel-cadium (NiCd) batteries. One advantage of the NiMH battery is that higher energy densities can be reached. Practically, this means that more energy can be obtained with the same volume because the metal hydride electrode can be made smaller. As with all battery technologies today, one important driver for development is obtaining higher energy densities. One way of increasing energy densities is to make the separator thinner, so that it takes up less space within the battery. To make the separator thinner, one must balance numerous competing factors. For example, the strength of the separator is important during cell manufacture and strength may be easily obtained by increasing, for example, thickness; but increased thickness can be detrimental to the cell energy density or the internal electrical resistance of the battery. Furthermore, the NiMH electrodes expand and contract during charging and discharging which creates a preference for a separator that will expand and contract with the electrodes. The proper balance of these factors is far from mere optimization, but instead, it requires tedious experimentation. Accordingly, there is a need for improved separators for NiMH batteries.
Several different types of separators have been suggested for use in alkaline battery systems, such as NiMH batteries. See: Besenhard, Ibid. xc2xa79.3.5, Tables 15-17. Those separators have been classified as: 1) nonwoven materials; 2) microporous materials; and 3) semipermeable materials. Of particular interest to the instant invention are the nonwoven materials. A nonwoven material (or fabric) is made from randomly-positioned fibers (e.g., a web) which are held together by mechanical interlocking, by fusing of the fibers, and/or by bonding the fibers (e.g., with a cementing medium). Web formation may be accomplished by various processes including, dry laying, wet laying, or extrusion of filaments onto a moving belt. Within the extrusion category, two processes include spunbonding (making spunbonded nonwovens) and melt-blowing (making melt-blown nonwovens). See: Turbak, A., Ed., Nonwovens: Theory, Process, Performance, and Testing, TAPPI Press, Atlanta, Ga. (1993). Chp 8, incorporated herein by reference. Spunbonded nonwovens are formed by filaments that have been extruded, drawn, and then laid on a continuous belt. Melt-blown nonwovens are formed by extruding molten polymer through a die, attenuating, via air or steam, the extruded filament and collecting them on a moving belt.
It is known to use a spunbond nonwoven as a separator in NiMH batteries. See: U.S. Pat. No. 5,830,604. It is known to use a melt-blown nonwoven as a battery separator. See: U.S. Pat. No. 5,962,161. It is known to use an SMS nonwoven in an alkaline battery system. See PCT Publication No. WO 00/41254. The foregoing separators, however, have not met the stringent demands necessary for use in a NiMH battery. Specifically, the foregoing nonwovens either have: pores which are too large, but have sufficient tensile and puncture strength (i.e., spunbond); or pores which are adequately sized, but have insufficient tensile and puncture strength (i.e., melt-blown); or a non-resilient or insufficiently resilient thickness (i.e., SMS).
Finally, since most nonwovens are made of polyolefins, which are inherently hydrophobic, they must be made hydrophilic so that the aqueous electrolyte will wet out the separator. Hydrophilic refers to the ability to xe2x80x9cwet-outxe2x80x9d a liquid. xe2x80x9cWet-outxe2x80x9d refers to the ability to cause a liquid (e.g., an aqueous solution) to penetrate more easily into, or spread over the surface of another material. Hydrophobic refers to the inability to xe2x80x9cwet-outxe2x80x9d a liquid.
Accordingly, there is a need for a wettable, resilient nonwoven separator having sufficient mechanical properties and sufficiently small pores for use in NiMH battery.
Preferably, the instant invention is to a battery separator for a nickel-metal hydride (NiMH) battery. The separator includes a wettable, resilient nonwoven web having two spunbond layers sandwiching a melt-blown layer. The web has a puncture strength of greater than 6 newtons, a tensile strength of greater than 200 newtons/meter, and an average pore size of less than 20 microns.